1. Field of the Invention
The present invention relates to a method for ion implantation used as one of methods for introducing an impurity in, for example, the field of the fabrication of semiconductor devices and to an apparatus for executing the method thereof.
This application is based on patent application No. Hei 9-088434 filed in Japan, the content of which is incorporated herein by reference.
2. Background Art
Ion implantation is used as one of the methods for introducing an impurity in the field of the fabrication of semiconductor devices. Ion implantation technology is widely used due to the major advantage that the dose of impurity atoms introduced into a target can be controlled very precisely by monitoring the integrated value of the injecting ionic current. The other advantages of ion implantation techniques is that the introduction of impurity atoms can be implemented without being hindered by a thin oxide layer formed on the target surface, and that doping can be carried out over a considerably wide target area under a controlled concentration and a controlled injection depth profile.
An example of ion implantation apparatuses is disclosed in Japanese Patent Application, First Publication, No. Hei 2-18851.
FIGS. 6 and 7 illustrate the conventional apparatus for ion implantation disclosed in the above Patent Application. Reference numeral 1 represents a semiconductor wafer, reference numeral 2 represents a disc for supporting a plurality of wafers 1 and for rotating the wafers in the X direction, numeral 3 represents a slit formed passing right through the thickness of the disc 2 at an predetermined position, numeral 4 represents an rotating shaft of the disc 2, numeral 5 represents a disc-holder for supporting the rotating shaft 4 of the disc 2, and numeral 6 represents a motor for rotating the rotating shaft 4 at a predetermined speed. Furthermore, reference numeral 7 represents a lead screw connected to the disc holder 5 and numeral 8 represents a stepping motor for rotating the lead screw 7. The stepping motor 8 may rotate the lead screw 7 in both rotating directions by means of a driver 9. The disc-holder 5 moves in either directions Y1 and Y2 according to the reciprocating rotation of the stepping motor 8.
Ionic particles are introduced into the wafer 1 by scanning the ion beam on the surface of the wafer when the wafer 1 enters under the path of an ion beam B. The scanning of the ion beam is performed by combining a movement of the disc 2 in the X direction and a reciprocal movement of the disc holder 5 in directions Y1 and Y2. Furthermore, when the slit 3 enters in the path of ion beam B by the rotation of the disc 2, ion beam B passes through the slit to be admitted in an ion beam detector 10 disposed behind the disc 2. The details of the ion beam detector will be described hereinafter.
In a controller 11, a timing generator 15 generates a timing signal 15a when the generator 15 detects that the ion beam enters at a position to be admitted in the ion beam detector 10 as shown by an broken line arrow in FIG. 6, and the timing signal 15a is then transmitted to a CPU 16. Reference numeral 12 represents a sample holding amplifier for sampling an ion beam current according to a command signal 16a generated by the CPU 16 based on the timing signal 15,
and for supplying the sampled value of the ion beam current 12a to a V/F converter 13 while holding the sampled current 12a until the next sampling time. The V/F converter 13 converts the sampled value 12a into a frequency signal 13a.
A beam counter 17 then counts the frequency signal 13a and generates the counter signal 17a, which is then transmitted to CPU 16. Consequently, the CPU 16 obtains the number of ionic particles introduced in the wafer 1 as a dose. On the other hand, the frequency signal 13a is divided by a frequency divider 14 according to the control signal 16b of CPU 16, and the divided signals 14a is transmitted to the driver 9. The driver 9, then drives the stepping motor 8 at a rate proportional to the signal 14a. The rotational speed of the lead screw 7 (the scanning speed of the disc 2) is controlled to be proportional to the signal 14a.
The CPU 16 receives the signal 14a of the divider 14 and can obtain a number of scanning cycles and an amount of displacement of the disc-holder 5, etc. In a manner as described above, the CPU 16 controls the scanning speed of the disc 2 by generating the command signal 16b through the predetermined calculation procedures based on the signals 14a and 17a. For example, if the detected quantity of the ion beam is too large, the CPU controls so as to decrease the scanning speed of the disc 2.
However, a few problems were observed in the above conventional ion implantation apparatus. The first problem was observed when the slit 3 is deformed by damage or deterioration caused by hitting of the ion beam, as shown in FIG. 8, which shows an example in which a width of the slit 3 is extended at the central portion by ion hitting. In this case, the ion dose distribution was non-uniform in a wafer surface area, even though the ion beam current I is maintained at a constant value.
That is, if the slit 3 is deformed, for example, as shown in FIG. 8, an ion beam current I.sub.2 passing through the center part of the slit 3 is larger than ion beam currents I.sub.1 and I.sub.3 which pass through both end portions of the slit 3. As a result, the scanning speed for ion beam impinging on the position B' in the wafer 1 being set faster than that of the ion beam impinging on the positions A' or C'. However, since the quantity of ions in the ion beam B, that is, the current of the ion beam B is maintained constant, the dose at the central portion B' becomes small. That is, the first problem was that the deformation of the slit 3 produces non-uniform distribution of the dose in the wafer.
In the conventional ion implantation apparatus, as shown in FIG. 9, a second problem was observed in that the ion beam B projecting on the central portion of the wafer is apt to spread in a larger area than that of the ion beam projecting on the peripheral area of the wafer due to the effect of charge-up of the wafer surface. When the slit 3 is not deformed, the ion beam current I.sub.4 at the central portion becomes smaller than the ion beam current I.sub.5 at the peripheral areas. This results in setting the scanning speed at the central portion far lower than the necessary speed and produces a higher dose at the center portion of the wafer, because the ion beam current is maintained at a constant current.
That is, the second problem of the conventional apparatus is that it is not provided with a device to eliminate the effect of the charge-up of the wafer surface so that wafers with a non-uniform dose distribution are produced.